Semiconductor switching devices have long been utilized in a variety of electronic control applications. Semiconductor devices of the thyristor family, including silicon controlled rectifiers (SCRs) and TRIACS, are characterized by having two states, an on-state and an off-state, making them useful for electric power regulation, switching, and phase control. An SCR is a unidirectional PNPN semiconductor device having an anode, a cathode, and a gate. In the off-state, an SCR is a high-resistance, low-current circuit element at voltages up to its breakover voltage. A momentary positive pulse applied to the gate of an SCR will bias the device so that it switches to the on-state and becomes a low-resistance, high-current element. Once an SCR is in the on-state, it will remain in the on-state until the principal anode-to-cathode current drops below the holding current of the device.
A TRIAC is a three-terminal, bidirectional thyristor which functions to control power in an AC electric circuit. The electrical characteristics of a TRIAC can be compared to that of two SCRs connected in inverse parallel relationship. Thus, a TRIAC functions essentially the same in both directions as an SCR behaves in its forward direction. A TRIAC has a single gate, and it can be triggered to the on-state by a gate pulse of either polarity.
A five-layer, bidirectional Shockley diode is a two-terminal semiconductor device which functions essentially the same as an SCR, but in both directions. As voltage is applied to either terminal, the device acts to block current until the voltage reaches a breakover level, at which point the device turns on and allows current to flow. This type of device is relatively nonsensitive in that the switching current and the holding current required to maintain the device in its on-state are generally 5 to 500 milliamperes. Furthermore, the breakover voltage level of the device is relatively inflexible in that it depends on the resistivity of the semiconductor slice and the concentration gradient of the diffused ions. If the device is fabricated with resistivity chosen for low breakover voltage, the conductivity of the device is impaired at the point immediately after breakover, and the voltage-current curve has a positive slope up to the breakover voltage point, which normally is undesirable. The undesirable slope of the voltage-current curve can be reduced by decreasing the thickness of the semiconductor chip, but processing of thin semiconductor wafers becomes impractical due to handling and breakage problems.
In many electrical applications it is desirable to have a switching device with a bidirectional, low voltage, low current triggering response. Furthermore, it would be desirable to provide a breakover device that is relatively sensitive, that switches on fully when the breakover voltage is reached, and whose voltage breakover point is readily adjustable during fabrication of the device.